The invention relates to a method and an arrangement using the inertia of different sets of rotatable internal transmission components in a single or dual-clutch transmission for assisting an electric machine during engine start-up under predetermined conditions.
A hybrid vehicle comprises an internal combustion engine, which transmits torque to the driving wheels by means of a transmission provided with a gear box, and at least one electric machine which is electrically connected to an electric storage system and mechanically connected to the driving wheels.
Hybrid vehicles can be operated in a thermal operating mode, in which the torque is generated only by the internal combustion engine and where the electric machine may work as a generator for recharging the storage system. Alternatively, hybrid vehicles can be operated in electric operating mode, in which the engine is switched off and the torque is generated only by the electric machine working as a motor. A third operating mode is a combined operating mode, in which the torque is generated both by the engine and by the electric machine working as a motor. Furthermore, in order to increase the overall energy efficiency during all steps of decelerating, the electric machine may be used as a generator for a regenerative deceleration in which the kinetic energy possessed by the vehicle is partially converted into electricity, which is stored in the storage system, instead of being completely dissipated in friction within the brakes.
A suitable transmission for a hybrid vehicle comprising an internal combustion engine and an electric machine is shown in WO 2012/123171. This document relates to a hybrid drive of a motor vehicle, which comprises an automated manual transmission (AMT) with a dual clutch transmission having two coaxially arranged input shafts and a common output shaft. One of the input shafts of said hybrid drive can be connected to the drive shaft of an internal combustion engine and can be drivingly connected to the output shaft by means of an associated first group of selectively shiftable gearwheel sets and the other input shaft is drivingly connected to the rotor of an electric machine that can be operated as a motor and as a generator, and can be drivingly connected to the output shaft by means of an associated second group of selectively shiftable gearwheel sets. The gearwheel sets are arranged between one of the two input shafts and one of two countershafts that are drivingly connected to the output shaft by means of an output constant, every gearwheel set comprising a fixed gear fixedly arranged on the associated input shaft and a free wheel arranged on the corresponding countershaft. Per input shaft at least the two transmission-internal gearwheel sets with free wheels arranged on different countershafts are arranged in a common radial plane using a common fixed wheel. At least two free wheels arranged on one of the two countershafts can be coupled to each other by two transmission-internal gearwheel sets associated with different input shafts by means of a winding shift element. The two output constants are arranged in a common radial plane using a common output wheel arranged on the output shaft.
The electric machine can be disconnected from the driving wheels, whereby it can be used to start the internal combustion engine. In this way, the electric machine can assist or replace a starter motor for starting the engine. When the engine is “cold”, the engine requires the application of a relatively high torque (indicatively even 2-4 times higher than the cranking torque needed to start a “warm” engine) to the crankshaft, because due to the low temperature, the lubricant oil inside the engine is not very viscous and creates a high mechanical resistance. A traditional starter motor is able to apply a very high torque to the crankshaft of the engine by virtue of a very high gear-down ratio (for instance 1:10) between the starter motor and the engine crankshaft. The electric machine of a hybrid vehicle is normally mechanically connected to the crankshaft of the engine with a relatively low gear-down ratio (often unitary, i.e. 1:1, providing no gear-down). Therefore, the electric machine of a hybrid vehicle may not be capable of generating the very high cranking torque needed to start the engine when the engine is cold and/or when the ambient temperature is low. In this context, the term “cold” denotes temperatures around ambient temperature. “Low” ambient temperature is used to denote temperatures around or below freezing.
US 2002/117860 describes a solution to this problem, where it is suggested to mechanically couple the electric machine to the crankshaft of the engine with two different, automatically selectable transmission ratios: a first more geared-down ratio is selected during a step of starting the engine, while a second, more direct ratio (i.e. less geared-down ratio) is selected for the remaining use of the electric machine. However, this solution requires a complex construction because a servo-controlled gear box would have to be interposed between the electric machine and the crankshaft.
WO 2007/131838, US 2005/155803, US 2009/017988 and U.S. Pat. No. 6,354,974 disclose solutions where the electric machine is separated from the crankshaft of the engine by a clutch. In order to start the engine, the clutch is opened, the electric machine is “idled” and the clutch is closed with the electric machine rotating so as to also exploit the inertia (i.e. the kinetic energy) of the electric machine for making the engine rotate. However, such a “launched” starting mode may also not be sufficient to start an engine which has a high displacement and with a high number of cylinders. Furthermore, the “launched” starting mode allows to apply a high cranking torque to the crankshaft needed to start a “cold” engine, but which is redundant to start a “warm” engine; therefore, when the engine is “warm”, the “launched” starting mode may unnecessarily impose high mechanical stresses on the transmission components and unnecessarily determines a certain delay when starting the engine due to the time needed to “idle” the electric machine.
DE 196 45 943 describes starting the engine with a “launched” starting mode which includes making the electric motor rotate before mechanically connecting the electric machine to the engine when the engine is “cold”. When the engine is “warm” a conventional starting mode is used in which the electric machine is mechanically connected to the engine before it starts to rotate.
GB 2 413 998 describes starting the engine with a “launched” starting mode which includes making the electric machine rotate before mechanically connecting the electric machine to the engine. The launch (or crank) rotation speed, i.e. the rotation speed which should be reached by the electric machine before mechanically connecting the electric machine to the engine, is determined according to the temperature of the engine so that the launch rotation speed is higher when the engine is “cold”.
A problem with the above solutions is that the use of an “idled” or a “launched” starting mode may not be sufficient for cranking the engine under adverse operation conditions. A further problem is that the suggested solution cannot be applied to a vehicle provided with a dual-clutch transmission.
It is desirable to provide an improved method for starting an engine that solves the above problems.
In the subsequent text, the term “transmission arrangement” is intended to describe the parts of a transmission located between an internal combustion engine crankshaft and a drive shaft driving at least one pair of driven wheels. The transmission arrangement preferably, but not necessarily, includes a range gear connected to the drive shaft.
The term “engine” will be used to denote an internal combustion engine, while the term “motor” will be used to denote an electric machine. Such an electric machine will be capable of being operated both as a motor, for assisting the engine, and as a generator, for charging an energy storage or driving accessories. When operated as a motor, the electric machine can be used for cranking the engine, for assisting the engine or for electric drive only.
The term “energy storage” will be used to denote a high voltage battery used for cranking or propelling a hybrid vehicle using the electric machine. The term “starter battery” or “starting battery” is used for describing a separate battery (12/24 V) dedicated for cranking an engine, usually by means of a separate starter motor not to be confused with the electric machine defined above. In the text, the invention is referred to as a method for “starting” an internal combustion engine, wherein the starting operation includes “cranking” of the engine. However, for the purpose of the invention, these terms are considered interchangeable.
According to a preferred embodiment, the invention relates, according to an aspect thereof, to a method for starting an internal combustion engine of a hybrid vehicle. The vehicle comprises an engine provided with a crankshaft, which crankshaft is connectable to at least one pair of driving wheels via a transmission arrangement. The vehicle further comprises a transmission, which can be a single or dual-clutch transmission, connectable to the engine. The transmission comprises multiple rotatable components for transmitting torque from the engine to the driving wheels.
The transmission has at least one input shaft wherein at least one clutch is arranged to connect a corresponding input shaft to the engine. The transmission further comprises an output shaft and at least a first countershaft which are connectable to the at least one clutch by means of a number of manually and/or automatically selectable gears. An electric machine is mechanically connected to the countershaft downstream of the at least one clutch, so that said clutch is interposed between the electric machine and the engine.
During normal operation of the vehicle, when the vehicle is in motion, the electric machine is connected to the at least one clutch, depending on the currently selected gear. During this operation gears can be selected manually, by the driver, automatically, or by an electronic control unit (ECU) using a predetermined gear change strategy.
During an engine start-up, when the vehicle is normally stationary, the electric machine can be connected to the at least one clutch under the control of the ECU only.
The starting method comprises the steps of:                detecting at least one engine related parameter to determine whether the electric machine is to be assisted for cranking the engine;        opening at least one clutch to separate the electric machine from the engine;        controlling the transmission for separating the electric machine from the driving wheels;        selecting at least one of a number of sets of rotatable transmission components to be used for assisting engine start, based on the at least one detected engine related parameter,        engaging at least one gear, connecting the at least one selected set of rotatable transmission components to the at least one clutch;        operating the electric machine as a motor to rotate the at least one selected set of rotatable transmission components and to reach predetermined crank rotation speed; and        closing one of the clutches when the electric machine reaches the crank rotation speed, in order to start the engine.        
The term “crank rotation speed” is used to define a rotational speed of the electric machine that is sufficient for cranking the engine.
As stated above, the number of rotatable components in the single or dual-clutch transmission to be used for assisting engine start is selected depending on the at least one detected engine related parameter. The starting method is primarily intended for use in low ambient temperatures, low engine coolant temperatures, and/or when the electric motor torque available from the electric machine is deemed to be near or below the torque required to ensure a successful cranking of the engine. The available electric motor torque is dependent on the energy storage temperature and/or the state of charge (SOC) of the energy storage or high voltage battery. During a cold start, the energy storage temperature will be dependent on the ambient temperature.
When it is detected that the energy storage or high voltage battery used for cranking the engine may not be able to deliver sufficient electric power for cranking the engine, using the electric machine connected directly to one of the clutches, then it is determined that the electric machine requires assistance during cranking. If the engine is normally started by means of a separate starter motor acting on the engine flywheel, a detected condition (low starter battery voltage, low ambient temperature and/or low engine coolant temperature) can cause the electric machine to take over the function of the starter motor.
According to a first example, the method involves operating the electric machine and using the moment of inertia of a first set of rotatable components. The first set comprises the rotary masses of the electric machine, the countershaft, at least one input shaft and at least one clutch for assisting the electric machine when cranking the engine. The method according to the first example is carried out when the electric motor torque available from the electric machine is deemed to be near or below the torque required to ensure a successful cranking of the engine. As stated above, the available electric motor torque is dependent on the temperature and/or the state of charge (SOC) of the energy storage or high voltage battery. During a cold start, the energy storage temperature will be dependent on the ambient temperature. It is determined that the electric machine requires assistance during cranking if the ambient temperature, the energy storage temperature and/or the state of charge (SOC) of the energy storage has dropped below a first set of predetermined limits or is outside a first range of predetermined limits.
In a second example, an additional selected set of rotatable transmission components is used for cranking the engine. In this example, the method involves operating the electric machine and using the rotary masses of the electric machine, the countershaft, at least one first input shaft, at least one clutch, as well as an output shaft, for assisting the electric machine when cranking the engine. In this case, the output shaft with its associated gears forms a second set of rotatable components. The method according to the second example is carried out when the ambient temperature, the energy storage temperature and/or the state of charge (SOC) of the energy storage or starting battery has dropped below a second set of predetermined limits. The second set of predetermined limits is lower than the first set of limits or is outside a second range of predetermined limits. This indicates that the electric machine will require additional assistance during engine start. The assistance provided by the second example is greater than that provided by the first example, as it involves additional rotary masses.
According to a first alternative embodiment, the invention relates to a method for starting an internal combustion engine of a hybrid vehicle. The vehicle comprises an engine provided with a crankshaft, which crankshaft is connectable to at least one pair of driving wheels via a transmission arrangement. The vehicle further comprises a dual-clutch transmission connectable to the engine. The dual-clutch transmission comprises multiple rotatable components for transmitting torque from the engine to the driving wheels. The dual-clutch transmission has a first and a second input shaft of which the first input shaft is a solid shaft connected to a first clutch and the second input shaft is a hollow shaft connected to a second clutch. The second input shaft is co-axially supported in the first input shaft. The dual-clutch transmission further comprises an output shaft and at least a first countershaft which are connectable to the first clutch and the second clutch, respectively, by means of a number of manually and/or automatically selectable gears. An electric machine is mechanically connected to the countershaft downstream of the clutches, so that the clutches are interposed between the electric machine and the engine. The electric machine can be connected to one or both clutches by the actuation of one or two gears, respectively.
During normal operation of the vehicle, when the vehicle is in motion, the electric machine is connected to one of the clutches, depending on the currently selected gear. During this operation gears can be selected manually, by the driver, automatically, or by an electronic control unit (ECU) using a predetermined gear change strategy.
During an engine start-up, when the vehicle is normally stationary, the electric machine can be connected to one or both clutches under the control of the ECU only.
The starting method comprises the steps of:                detecting at least one engine related parameter to determine whether the electric machine is to be used for cranking the engine;        opening the clutches to separate the electric machine from the engine; and        separating the electric machine from the driving wheels;        
When the dual-clutch transmission has been separated from the engine and the driven wheels, the method according to the invention performs the further steps of:                selecting one of a number of sets of rotatable transmission components to be used for assisting engine start, based on the at least one detected engine related parameter,        engaging one gear, connecting the at least one selected set of rotatable transmission components to one clutch;        engaging a further gear, connecting at least the countershaft to the other clutch;        operating the electric machine as a motor to rotate the selected rotatable components and to reach a predetermined crank-rotation speed; and        closing one of the clutches when the electric machine reaches the crank-rotation speed, in order to start the engine.        
In a first example of the first alternative embodiment, only one selected set of rotatable transmission components is used for cranking the engine. In this example, the first gear is used for connecting the countershaft to the first clutch. In addition, the second gear is used for connecting the countershaft to the second clutch. When the electric machine is actuated, the countershaft will drive both the first clutch, together with the first input shaft, and the second clutch, together with the second input shaft, through the first gear and the second gear, respectively. These parts form a first set of rotatable components. When the electric machine reaches the predetermined crank rotation speed the second clutch is closed to crank and start the engine. In this example, the momentum from the first clutch and the first input shaft will be transferred to the countershaft via the second gear, providing a predetermined gear ratio multiplier. The torque from the electric machine will be transferred to the second clutch, the second input shaft and the output shaft via the first gear. The electric machine is deemed to require assistance by a first set of selected set of rotatable transmission components during cranking if at least one detected engine related parameter has dropped below a first set of predetermined limits or is outside a first range of predetermined limits.
It is also possible to close the first clutch for cranking the engine, but in order to utilize the optimum gear ratio for transferring the inertia of the rotating transmission components to the engine during start, the second clutch is selected.
According to the first example as described above, the electric machine is separated from the driving wheels by disconnecting the first and second input shafts from the output shaft.
In a second example of the first alternative embodiment, an additional selected set of rotatable transmission components is used for cranking the engine. In this example, the method involves operating the electric machine and using the rotary masses of the electric machine, the countershaft, the first and second input shafts, the first and second clutches, as well as the output shaft, for assisting the electric machine when cranking the engine. In this case, the output shaft with its associated gears forms a second set of rotatable components. The method according to the second example is carried out when the electric machine is deemed to require assistance by the first and an additional, second set of selected set of rotatable transmission components during cranking.
The method according to the second example is used when the at least one detected engine related parameter is below a second set of predetermined limits or is outside a second range of predetermined limits, lower than said first set of limits. This indicates that the electric machine will require additional assistance during engine start. The assistance provided by the second example is greater than that provided by the first example, as it involves additional rotary masses.
In the second example, the second gear is used for connecting the countershaft to the second clutch. In addition, a base gear is used for connecting the countershaft and the output shaft to the first clutch. When the electric machine is actuated, the countershaft will drive the first clutch together with the first input shaft and the output shaft through the base gear. At the same time, the countershaft will drive the second clutch and the second input shaft through the second gear. When the electric machine reaches the predetermined crank rotation speed the first clutch is closed to crank and start the engine. In this example, the momentum from the second clutch and the second input shaft and the torque from the electric machine will be transferred to the first clutch, the second input shaft and the output shaft via the base gear, providing a further predetermined gear ratio multiplier.
It is also possible to close the second clutch for cranking the engine, but in order to utilize the optimum gear ratio for transferring the inertia of the rotating transmission components to the engine during start, the first clutch is selected.
According to the second example as described above, the electric machine is separated from the driving wheels by disconnecting the output shaft from a range gear interposed between the output shaft and the driving wheels.
According to a second alternative embodiment, the invention relates to a method for starting an internal combustion engine of a hybrid vehicle comprising a single-clutch transmission connectable to the engine. The single-clutch transmission comprises multiple rotatable components for transmitting torque from the engine to the driving wheels. The single-clutch transmission has a first input shaft which is a solid shaft connected to a first clutch. The single-clutch transmission further comprises an output shaft and at least a first countershaft which are connectable to the first clutch by means of a number of manually and/or automatically selectable gears. An electric machine is mechanically connected to the countershaft downstream of the clutch, so that the clutch is interposed between the electric machine and the engine. The electric machine can be connected to the clutch by the actuation of one or two gears, respectively.
During normal operation of the vehicle, when the vehicle is in motion, the electric machine is connected to the clutch via the currently selected gear. During this operation gears can be selected manually, by the driver, automatically, or by an electronic control unit (ECU) using a predetermined gear change strategy.
During an engine start-up, when the vehicle is normally stationary, the electric machine can be connected to the clutch under the control of the ECU only.
The starting method comprises the steps of:                detecting at least one engine related parameter to determine whether the electric machine is to be used for cranking the engine;        opening the clutch to separate the electric machine from the engine; and        separating the electric machine from the driving wheels;        
When the single-clutch transmission has been separated from the engine and the driven wheels, the method according to the invention performs the further steps of:                selecting one of a number of sets of rotatable transmission components to be used for assisting engine start, based on the at least one detected engine related parameter;        engaging one gear, connecting the at least one selected set of rotatable transmission components to the clutch;        operating the electric machine as a motor to rotate the selected rotatable components and to reach a predetermined crank-rotation speed; and        closing the clutch when the electric machine reaches the crank-rotation speed, in order to start the engine.        
In a first example of the second alternative embodiment, only one selected set of rotatable transmission components is used for cranking the engine. In this example, the second gear is used for connecting the countershaft to the first clutch. When the electric machine is actuated, the countershaft will drive a first set of rotatable components, comprising the rotary masses of the electric machine, the countershaft, the first input shaft and the first clutch, for assisting the electric machine when cranking the engine. When the electric machine reaches the predetermined crank rotation speed the first clutch is closed to crank and start the engine. In this example, the momentum will be transferred from the countershaft to the first clutch and the first input shaft via the second gear, providing a predetermined gear ratio multiplier. The torque from the electric machine will be transferred by the same transmission components. The electric machine is deemed to require assistance by a first set of selected set of rotatable transmission components during cranking if at least one detected engine related parameter has dropped below a first set of predetermined limits or is outside a first range of predetermined limits.
According to the first example as described above, the electric machine is separated from the driving wheels by disconnecting the first input shaft from the output shaft.
In a second example of the second alternative embodiment, a first and a second set of rotatable transmission components are used for cranking the engine. In this example, the second gear is used for connecting the countershaft to the first clutch. In this example, the method involves operating the electric machine and using the rotary masses of the electric machine, the countershaft, the first input shaft, the first clutch, as well as the output shaft, for assisting the electric machine when cranking the engine. In this case, the output shaft with its associated gears forms a second set of rotatable components. The method according to the second example is carried out when the electric machine is deemed to require assistance by an additional, second set of selected set of rotatable transmission components during cranking. The second set of predetermined limits is lower than the first set of limits or is outside a first range of predetermined limits, indicating that the electric machine will require additional assistance during engine start. The assistance provided by the second example is greater than that provided by the first example, as it involves additional rotary masses.
In this second example, a base gear is used for connecting the countershaft and the output shaft to the input shaft and the first clutch. When the electric machine is actuated, the countershaft will drive the first clutch, together with the first input shaft, and the output shaft through the base gear. When the electric machine reaches the predetermined crank rotation speed the first clutch is closed to crank and start the engine. In this example, the momentum from the countershaft and the output shaft and the torque from the electric machine will be transferred to the first clutch via the base gear, providing a predetermined gear ratio multiplier.
According to the second example as described above, the electric machine is separated from the driving wheels by disconnecting the output shaft from a range gear interposed between the output shaft and the driving wheels.
According to the method and any one of the examples given above, at least the temperature and/or the state of charge of an energy storage used for cranking the engine is detected. The energy storage is preferably a high voltage main storage battery or energy storage used for propulsion of the vehicle. According to one example, the temperature of the energy storage or the state of charge (SOC) of the energy storage are detected to determine whether the electric motor torque available from the electric machine is sufficient for cranking the engine. When the detected temperature and/or state of charge (SOC) is below at least one predetermined limit, the electric machine is operated as a starter motor and is given different levels of assistance using a variable number of rotary components in the dual-clutch transmission during the cranking of the engine. The operation of the electric machine and the dual-clutch transmission is controlled by an electronic control unit (ECU). At least one predetermined limit for detected temperature and SOC can be stored in the ECU, where the degree of assistance provided by the dual-clutch transmission is decided by the current temperature and/or SOC.
According to a further example, it is determined that the electric machine is to be assisted during the cranking of the engine when it is detected that the ambient temperature is below a predetermined limit. The torque required for cranking the engine is dependent on the engine friction, which in turn is primarily dependent on the ambient temperature. According to a further example, it is determined that the electric machine is to be assisted during the cranking of the engine when it is detected that the engine coolant temperature is below a predetermined limit. The amount of assistance required for the electric machine can be determined by each of the above detected parameters individually or in combination. For instance, if the temperature and/or the SOC of the energy storage is below a first predetermined limit, the ECU can decide that a level of assistance according to the first example described above is sufficient for cranking the engine. However, a further condition to be checked can be the ambient temperature and/or the engine coolant temperature. If one or both of these parameters are below a predetermined limit, then The ECU can decide that an additional level of assistance according to the second example described above must be provided for cranking the engine.
An additional parameter that can be controlled in connection with the engine start is the rotation speed of the electric machine. The ECU can determine the crank rotation speed according to the at least one engine related parameter. For instance, additional torque can be provided for engine start by increasing the crank rotation speed if one or more detected engine related parameters are below a set of predetermined limit values. The selected crank rotation speed rotation speed can be used for determining a closing speed of the closing clutch used during the cranking operation.
The invention further relates to a vehicle comprising an internal combustion engine and a single or dual-clutch transmission, which engine is started by means of the method as described above. The invention allows the degree of assistance provided by the transmission during a start-up procedure to be controlled by adapting the number of rotational components in the single or dual-clutch transmission. The degree of assistance is determined by detecting one or more engine related parameters and comparing detected values with predetermined limit values, as described above.
The present invention also relates to a computer program, computer program product and a storage medium for a computer all to be used with a computer for executing the method as described in any one of the above examples.